The present invention relates in general to an inexpensive and reliable muting circuit for audio amplifiers with differential inputs and more specifically to an audio system having a main output section and an auxiliary output section wherein a differential amplifier in the auxiliary section is muted separately from the main output section.
High performance automotive audio systems frequently employ a power amplifier located remotely from the main audio unit in the dashboard. By distributing the system components throughout the vehicle, the limited dashboard space is conserved and the amount of heat that would otherwise be generated in the main unit is reduced.
In such a distributed system, audio signals supplied from the main unit to the remote power amplifier are characterized by a low voltage level. Because of the distance traversed by the signal lines between the main unit and the power amplifier and because of the low level of the audio signals, the signal which is input to the power amplifier tends to include an undesirable amount of noise signal.
To solve the problem of conducting low level signals within the relatively noisy environment of an automobile, the audio signal has been conducted between the main unit and the power amplifier as a differential signal. Thus, a pair of electrical lines are used to conduct a signal and an inverted signal (i.e., identical but 180.degree. out of phase). The signal and the inverted signal are provided to a differential amplifier which amplifies the difference between the two signals. The output of the differential amplifier equals the original signal multiplied by a constant and does not include noise picked up by the electrical lines since each line receives the same noise signal which then cancels out in the differential amplifier (known as the common mode rejection of the differential amplifier).
Audio systems typically employ muting to inhibit action by the power amplifier during times that no valid audio signal is being output by the main unit. Such times include periods when a radio is being scanned-tuned, a tape player is being rewound, or during power-up when loud pops caused by power-on transients occur. Such muting is especially desirable in auxiliary head phone units which tap off the main system to allow private listening or listening at increased volume without inconveniencing other passengers in a vehicle.
Power amplifiers are typically constructed using integrated circuits which are available from many manufacturers. Some integrated circuits are available which are internally muted under control of a mute signal supplied to a pin of the integrated circuit. However, internally muted integrated circuits may not be available for a particular application depending on desired power capacity and other requirements. Depending upon production volume, it may not be possible to obtain a custom made integrated circuit for a particular application so that a standard unmutable integrated circuit might be the only option. Therefore, a means is needed for externally muting an audio amplifier having differential inputs.